The present invention relates to an electrode cartridge with a strain gauge which is applied to measurement of an internal stress for a film of plating and a system for implementing the measurement with the electrode cartridge.
Of late years plating has been extensively applied to a surface of metal. The smaller becomes the size of an object to which plating is applied, the higher requirements have been imposed on a thinner film of plating and its performance. Especially in the field of semiconductors, it has come to require that a thickness and width of plating be controlled to be as small as some nanometers.
It is necessary to optimize conditions of electro deposition such as composition of a plating liquid, a current density, a speed of agitation and a temperature of plating liquid so as to constantly obtain a best film of plating. It sometimes happens in a generally known method for plating that characteristics of a film of plating drift as composition of a plating liquid progressively departs from its original ones, even if optimized conditions are established at starting of plating. This leads to a necessity that conditions be controlled by continuous monitoring of a plating bath so that stable characteristics of a film of plating may be maintained. One example of monitoring conditions of a plating bath is to measure an internal stress of a film of plating (hereinafter shortly referred to as “internal plating stress” if necessary). A conventional apparatus for measuring an internal stress of a film of plating includes an apparatus using a spiral stress meter. Description is given of this apparatus (hereinafter referred to as “spiral plating stress meter” if necessary).
As shown in FIG. 7, a spiral plating stress meter 50 holds a test sample 51 of a spirally formed member around both a support shaft 52 with a clamp 53 and a rotational shaft 57 with a clamp 54. An inner surface (back surface) of the test sample 51 is coated with fluorescent resin so that a film of plating is formed only on its outer surface (front surface). The support shaft 52 is a cylindrical member having a hollow portion. The clamp 53 connected to a lower end portion of the support shaft 52 holds an upper end portion of the test sample 51. The rotational shaft 57 is a slender pole, which rotatably penetrates the hollow portion of the support shaft 52. An upper end portion of the rotational shaft 57 is connected to a pointer of a transducer 56 and its lower end portion to the clamp 54. In this way, torsion of the test sample 51 is transmitted to the rotational shaft 57, which produces a rotation of the pointer of the transducer 56.
When the spiral plating stress meter 50 is placed in a tank filled with a plating liquid and a current is supplied between the meter 50 and an anode plate by a power supply (not shown), a front surface of the test sample 51 is plated, which creates an internal stress in the test sample 51 and a resulting rotation of the rotational shaft 57. The transducer 56 transforms the rotation into a torsion angle (α), so that the meter 50 measures an internal plating stress (σ), which can be calculated by an expression (1).σ=(2k/pt)×(α/d)  (1)
where k is a spiral constant (mm·N/deg), p a pitch of spiral (mm), t a thickness of spiral plate (mm), α a torsion angle (deg) and d a thickness of plating (mm).
Although the spiral plating stress meter 50 is able to measure an internal stress of plating with relatively high accuracy, it is difficult to apply the meter 50 for measurement of a minute stress because it is necessary to read a pointer of the transducer 56 with the naked eye. Also it is burdensome for a person in charge when he reads the pointer at predetermined intervals so as to monitor a change in internal stress, which the spiral plating stress meter 50 is able to provide real time. Furthermore, because the pointer of the transducer 56 is read by the person with the naked eye, there is another problem that an error resulting from reading occurs.
As reading is performed for the pointer of the transducer 56 with the naked eye in the spiral plating stress meter 50, it is necessary for the pointer to displace so much that the displacement is visible to the naked eye. The smaller is stiffness of the test sample 51, the more rotational displacement makes the pointer of the transducer 56. Accordingly, the thinner is the test sample 51, the smaller stress it is theoretically possible to measure. However, there has been a problem that it is impossible to accurately measure an internal stress of a film of plating if the stiffness of the test sample 51 is decreased (the thickness of test sample 51 is reduced). This attributes to the fact that deformation of the test sample 51 releases the internal stress of the film of plating, thereby giving a misleading smaller value. In other words, the spiral plating stress meter 50 has limitations in terms of the accuracy of measurement for an internal stress of a film of plating.
On the other hand, another method for measuring an internal stress of a film of plating has been studied using a principle of strain gauge, which is reported in a non-patent document, Yutaka Tsuru et al. Development of the Measuring System for Mean Internal Stress in Nickel Film Plated on Copper Substance page 780-784 DENKI KAGAKU Vol. 60 No. 9, September 1992.
This method uses a strain gauge which is attached to a back surface of a cathode plate. The strain gauge measures distortion (minute deformation) of the cathode plate which results from an internal stress of a film of plating. In this way, the method calculates the internal stress based on the distortion.
FIG. 8 is a sectional view illustrating a distribution of voltage in a plating tank according to a conventional apparatus for measuring an internal stress of a film of plating using a strain gauge. A conventional apparatus 60 for measuring an internal stress of a film of plating includes a plating tank 61, a cathode plate C, an anode plate A, a shield plate 62. The plating tank 61 is filled with a plating liquid. The cathode plate C and the anode plate A are disposed so that these two plates mutually confront with a predetermined spacing. The shield plate 62, which has a through hole 62a of a predetermined size, is interposed between the cathode plate C and the anode plate A. A strain gauge HG is attached to a back surface of a plating section of the cathode plate C.
An accurate measurement for an internal stress of a film of plating with the apparatus 60 requires a uniform thickness of the film of plating, which is formed on the cathode plate C as an object to be plated. As shown in FIG. 8, the apparatus 60 is accordingly adapted to provide equipotential lines which are parallel with the cathode plate C. This is realized by not only placing the shield plate 62 having the through hole 62a, but also adjusting appropriately a spacing between the cathode plate C and the shield plate 62 as well as a spacing between the shield plate 62 and the anode plate A. In this way, the distribution of voltage within the tank is properly adjusted, so that it is possible to form a uniform thickness of a film of plating.
Japanese Published Patent Application 2000-002598 filed by the present applicant is listed here for information, which discloses an apparatus for measuring an internal stress of a film of plating.
However, in order to acquire a uniform thickness of a film of plating, it has been required to adjust (1) the spacing between the cathode plate C and the shield plate 62 as well as the spacing between the anode plate A and the shield plate 62, (2) a ratio of width between the through hole 62a and the cathode plate C and (3) geometrical relationship in a direction of height between the through hole 62a and the plating section of the cathode plate C. When an adjustment is manually done, it has sometimes taken much time or it has been sometimes impossible to obtain a uniform thickness of a film of plating due to a poor adjustment. This has occurred oftener when a measurement is made for an internal stress of a very thin film of plating.
Even if the adjustments such as (1) to (3) are successfully carried out, it has sometimes occurred that the width of the anode plate A varies due to its dissolution in continuing plating. This leads to a change in equipotential lines, thereby disarranging their parallelism relative to the cathode plate C. In this way, there has been a problem that a uniform thickness of a film of plating can not be obtained and an internal stress can not be accurately measured accordingly.
When a power supply is connected to both cathode plate C and anode plate A to supply current, hydrogen ions (H+) collect on a surface of the cathode plate C, forming pits (recesses) on a film of plating. If a large number of pits are created, it is impossible to accurately measure an internal stress of a film of plating. To solve this problem, it is necessary to prepare a device for removing the hydrogen ions, which collect on the film of plating. Methods for removing the hydrogen ions include one for providing vibration for the cathode plate C and another for blowing off the hydrogen ions by spraying the cathode plate C with bubbles. However, the method using vibration has had an adverse effect on measurement of strain gauge to create noises. On the other hand, the method using bubbles has sometimes affected conditions of a film of plating due to a variation in position of bubbles injected. In this way, when an adjustment is manually made for the geometrical relationship between the cathode plate C and injection holes, it has been difficult to guarantee the repeatability of a film of plating due to a fine deviation of the geometrical relationship, which results in degradation of accuracy for measurement of an internal stress of a film of plating.